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Project Progress

“Formation of Amines (RNH₂) and the Cyanide Anion (CN_-) in Electron-Irradiated Ammonia-Hydrocarbon Interstellar Model Ices”

The present laboratory study simulated cosmic-ray-induced grain chemistry of nitrogen-bearing organic molecules in interstellar and cometary ices. Model ices of ammonia (NH₃)–methane (CH₄) were prepared and irradiated at 10 K under contamination-free, ultrahigh vacuum conditions with energetic electrons generated in the track of galactic cosmic-ray particles. The radiolysis-induced processing of nitrogen-bearing molecules was then monitored on line and in situ by a Fourier transform infrared spectrometer and a quadrupole mass spectrometer during the irradiation phase and subsequent warm-up phases. The analogous processing was also achieved in ammonia (NH₃) and six hydrocarbon (CnH₂n+2; n=1–6) ices. The formation of cyanide anion (CN−) was commonly observed in both ices at 10 K, the temporal column density fit of which traced back the involvement of methylamine (CH3NH2)-based intermediates. Traces of CH₃NH₂ were evident at about 110 K through thin ammonia matrices in sublimation. From the point of radiative transfer, we further constrain the formation mechanism of amino acetonitrile (NH₂CH₂CN) on icy grains of Sgr B2(N) under a cosmic-ray-induced photon field.

“Laboratory Studies on the Formation of Formic Acid in Interstellar and Cometary Ices”

Mixtures of water (H₂O) and carbon monoxide (CO) ices were irradiated at 10K with energetic electrons to simulate the energy transfer processes that occur in the track of galactic cosmic-ray particles penetrating interstellar ices. We identified formic acid (HCOOH) through new absorption bands in the infrared spectra at 1690 and 1224 cm−1 (5.92 and 8.17μm, respectively). During the subsequent warm-up of the irradiated samples, formic acid is evident from the mass spectrometer signal at the mass-to-charge ratio, m/z = 46 (HCOOH+) as the ice sublimates. The detection of formic acid was confirmed using isotopically labeled water-d2 with carbon monoxide, leading to formic acid-d2 (DCOOD). The temporal fits of the reactants, reaction intermediates, and products elucidate two reaction pathways to formic acid in carbon monoxide–water ices. The reaction is induced by unimolecular decomposition of water forming atomic hydrogen (H) and the hydroxyl radical (OH). The dominating pathway to formic acid (HCOOH) was found to involve addition of suprathermal hydrogen atoms to carbon monoxide forming the formyl radical (HCO); the latter recombined with neighboring hydroxyl radicals to yield formic acid (HCOOH). To a lesser extent, hydroxyl radicals react with carbon monoxide to yield the hydroxyformyl radical (HOCO), which recombined with atomic hydrogen to produce formic acid. Similar processes are expected to produce formic acid within interstellar ices, cometary ices, and icy satellites, thus providing alternative processes for the generation of formic acid whose abundance in hot cores such as Sgr-B2 cannot be accounted for solely by gas-phase chemistry.

“Formation of Benzene in the Interstellar Medium”

Polycyclic aromatic hydrocarbons (PAHs) and related species have been suggested to play a key role in the astrochemical evolution of the interstellar medium, but the formation mechanism of even their simplest building block, the aromatic benzene molecule, has remained elusive for decades. We demonstrated in cros¬sed molecular beam experiments combined with electronic structure and statistical calculations that benzene (C₆H₆) can be synthesized via the barrierless, exoergic reaction of the ethynyl radical (C₂H) and 1,3-butadiene (H₂CCHCHCH₂) under single collision conditions. This reaction portrays the simplest representative of a whole new reaction class in which aromatic molecules with a benzene core can be formed from acyclic precursors via barrier-less reactions of ethynyl radicals with substituted 1,3-butadiene molecules. Novel gas–grain astrochemical models imply that this low temperature route controls the synthesis of the very first aromatic ring from acyclic precursors in cold molecular clouds such as in TMC-1. Rapid, subsequent barrier-less reactions of benzene with ethynyl radicals can lead to naphthalene-like structures thus effectively propagating the ethynylradical mediated formation of aromatic molecules in the interstellar medium.

“Mechanistical Studies on Formamide Production within Interstellar Ice Analogs”

Formamide, H₂NCHO, represents the simplest molecule containing the peptide bond. Conse¬quently, the formamide molecule is of high interest as it is considered as an important precursor in the abiotic synthesis of amino acids, and thus significant to further prebiotic chemistry, in more suitable environments. Previous experiments have demonstrated that formamide is formed under extreme conditions similar throughout the interstellar medium via photolysis and the energetic processing of ultra cold interstellar and Solar System ices with high energy protons; however no clear reaction mechanism has been identified. Utilizing a laboratory apparatus capable of simulating the effects of galactic cosmic radiation on ultra low temperature ice mixtures, we have examined the formation of formamide starting from a variety of carbon monoxide (CO) to ammonia (NH₃) ices of varying composition. Our results suggest that the primary reaction step leading to the production of formamide in low temperature ices involves the cleavage of the nitrogen–hydrogen bond of ammonia forming the amino radical (NH₂) and atomic hydrogen (H), of which the latter contains excess kinetic energy. These suprathermal hydrogen atoms can then add to the carbon–oxygen triple bond of the carbon monoxide (CO) molecule, overcoming the entrance barrier ultimately producing the formyl radical (HCO). From here, HCO may combine without an entrance barrier with the neighboring amino radical if the proper geometry for these two species exists within the matrix cage.

“On the Interaction of Methyl Azide (CH₃N₃) Ices with Ionizing Radiation: Formation of Methanimine (CH2NH), Hydrogen Cyanide (HCN), and Hydrogen Isocyanide (HNC)”

Methyl azide (CH₃N₃) might be a potential precursor in the synthesis of prebiotic molecules via nonequilibrium reactions on interstellar ices initiated by energetic galactic cosmic rays (GCR) and photons. Here, we investigate the effects of energetic electrons as formed in the track of cosmic ray particles and 193 nm photons with solid methyl azide at 10 K and the inherent formation of methanimine (CH₂NH), hydrogen cyanide (HCN), and hydrogen isocyanide (HNC). We present a systematic kinetic study and outline feasible reaction pathways to these molecules. These processes might be also important in solar system analogue ices. (HCOOH) was found to involve addition of suprathermal hydrogen atoms to carbon monoxide forming the formyl radical (HCO); the latter recombined with neighboring hydroxyl radicals to yield formic acid (HCOOH). To a lesser extent, hydroxyl radicals react with carbon monoxide to yield the hydroxyformyl radical (HOCO), which recombined with atomic hydrogen to produce formic acid. Similar processes are expected to produce formic acid within interstellar ices, cometary ices, and icy satellites, thus providing alternative processes for the generation of formic acid whose abundance in hot cores such as Sgr-B2 cannot be accounted for solely by gas-phase chemistry. [Alfredo Quinto-Hernandez and Alec M. Wodtke, Chris J. Bennett, Y. Seol Kim, and Ralf I. Kaiser]

“Interaction of Adenine with Ionizing Radiation: Mechanistical Studies & Astrobiology Implications”

The molecular inventory available on the prebiotic Earth was likely derived from both terrestrial and extraterrestrial sources. A complete description of which extraterrestrial molecules may have seeded early Earth is therefore necessary to fully understand the prebiotic evolution which led to life. Galactic cosmic rays (GCRs) are expected to cause both the formation and destruction of important biomolecules—including nucleic acid bases such as adenine—in the interstellar medium within the ices condensed on interstellar grains. The interstellar ultraviolet (UV) component is expected to photochemically degrade gas-phase adenine on a short timescale of only several years. However, the destruction rate is expected to be significantly reduced when adenine is shielded in dense molecular clouds or even within the ices of interstellar grains. Here, biomolecule destruction by the energetic charged particle component of the GCR becomes important as it is not fully attenuated. Presented here are results on the destruction rate of the nucleobase adenine in the solid state at 10 K by energetic electrons, as generated in the track of cosmic ray particles as they penetrate ices. When both UV and energetic charged particle destructive processes are taken into account, the half-life of adenine within dense interstellar clouds is found to be ~6 Myr, which is on the order of a star-forming molecular cloud.We also discuss chemical reaction pathways within the ices to explain the production of observed species, including the formation of nitriles (R–C≡N), epoxides (C–O–C), and carbonyl functions (R–C=O). [N.L. Evans, S. Ullrich, C.J. Bennett, R.I. Kaiser]

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